The attacks of Sept. 11, 2011 did more than propel American troops into Afghanistan and Iraq. They also affected a profound change in how the country conducts warfare.

As counterterrorism has become the preeminent goal, the military has relied increasingly on covert operations conducted by the Central Intelligence Agency or by an array of secretive forces designated broadly as Special Operations. That shift has introduced a new model of warfare, one that is characterized by unmanned drone strikes or targeted missions that occur outside of public awareness and often without explicit Congressional approval.

The CIA has recently expanded its campaign of targeted missile strikes into Yemen and Somalia, where a branch of Al Qaeda and the terrorist organization Al-Shabab are ascendant. Such operations are sanctioned under an Authorization for Use of Military Force, passed in the days following Sept. 11, 2011, that gives the president broad powers to deploy force against individuals responsible for Sept. 11 or people who harbor those responsible.

CIA director David Petraeus, who formerly commanded U.S. forces in Afghanistan, underscored in his Senate confirmation hearings how the military's operations and the CIA's operations have become intertwined -- something also attested to by his own transition.

One of the major developments since 9/11 has been the establishment of this network, in many cases led by the Joint Special Operations Command of the military, but with very, very good partnering with elements of the Central Intelligence Agency, other elements of the intelligence community, Petraeus said.

Since Sept. 11, the CIA has undergone a sweeping transformation. While the organization has always struck a balance between intelligence gathering and field operations, killing terrorists has taken on an outsize role. Drone strikes have killed more than 2,000 militants and civilians as the CIA's Counterterrorism Center has grown from 300 employees on the day of the attacks to more than 2,000, according to the Washington Post.

We're seeing the CIA turn into more of a paramilitary organization without the oversight and accountability that we traditionally expect of the military, Hina Shamsi, the director of the National Security Project of the American Civil Liberties Union, told the Post.

An anonymous former CIA official was more blunt, saying the agency's metamorphasis has been nothing short of a wonderment as it became one hell of a killing machine.

Those are almost the same words used by John Nagl, a past counterinsurgency adviser to Petraeus, who described Joint Special Operations Command, a subset of Special Operations Command that has taken a leading role in hunting terrorists, as an almost industrial-scale counter-terrorism killing machine.

The American public got a glimpse of this when a team of Navy SEALs swooped into Osama bin Laden's compound, and subsequently when a helicopter bearing 30 Special Operations Troops -- 22 of them SEALs -- was shot down in Afghanistan. But such clandestine missions are not confined to countries like Iraq and Afghanistan, where the military has a substantial troop presence. Special Operations forces are active in as many as 70 different countries, a number that could increase to 120 by year's end. Special Operations Command employs 60,000 people, up from about 37,000 in the early 1990's, and it has seen its budget increase from $2.3 billion to $9.8 billion in the wake of Sept. 11, 2001.

A Washington Post investigation highlighted the prominent role of Joint Special Operations Command, or JSOC, in prosecuting a global war on terror. JSOC has killed more militants than the CIA while imprisoning and interrogating more than ten times as many suspected terrorists. But while the organization has 1,800 troops prior to 9/11 to as many as 25,000, it has remained obscured from public view.

We're the dark matter, a Navy SEAL told the Post. We're the force that orders the universe but can't be seen.